Understanding the molecular mechanisms underlying the control of gene expression is a major objective in biology. The cII and cIII genes of phage lambda are key regulatory functions essential for the lysogenic pathway. The activity of both proteins is controlled, post transcriptionally, by the host, at the level of protein synthesis and degradation. The long-term objective of this research program is to understand, in molecular terms, the host-phage interactions in gene regulation and the phage life cycle. The cIII gene codes for a small peptide that most probably acts as an antiprotease extending the half-life of the CII protein. It was recently found to regulate a number of host genes, some of which belong to the heat-shock regulon. This research will aim to identify the host genes interacting with cIII gene function emphasizing the role of proteases in regulatory circuits. It is expected that this research will uncover novel regulatory proteases that recognize specific protein structural motifs and may lead to the discovery of E. coli antiprotease activities. Experimental systems for the selection and study of mutations in host and phage functions have been established. Host genes affecting CIII expression and stability will be isolated and characterized. Their importance in E. coli and phage lambda, regulatory processes will be investigated. A lacZ'-CIII bifunctional protein, carrying only 24 amino acid residues of CIII, greatly simplifies the search for genes regulating CIII activity and the in vivo and in vitro study of CIII proteolysis. Proteins specifically bound to CIII will be isolated and characterized. The presence of antiproteases in E. coli will be addressed by several new selection procedures. Finally, a model system for the identification and molecular characterization of protein domains that serve as targets for regulatory degradative proteases will be established.